A known used method for depositing thin films of materials onto substrates is cathode sputtering. The process involves vaporizing a target material by ion bombardment. The target usually forms a part of a cathode assembly placed in a chamber which is evacuated. An inert gas such as argon fills the chamber. An electric field is applied between the cathode assembly and an anode positioned within the chamber and electrons are ejected from the surface of the cathode. The inert gas is ionized by collision with the ejected cathode electrons to form positive gas ions which are attracted to the cathode surface. Particles of the target material are dislodged when the positive ions impact the target. The trajectory of these target particles is such that they traverse the enclosure and deposit as a thin film onto a positioned substrate.
As a result of efforts, in part, to increase and intensify deposition rates of target materials onto substrates, it is known to use magnetron sputtering in which a magnetic field is formed over the target surface. Known magnetically enhanced sputtering devices have in common a closed tunnel of magnetic lines of force over the sputtering surface formed by a magnetic system situated behind the sputtering surface, the tunnel serving to increase the ionization probability. The magnetic field formed is a closed loop over the surface of the target and is superimposed on the electric field. The magnetic field deflects and traps the electrons ejected or emitted from the surface of the target cathode so that they move in confined paths and are trapped in confined spaces above the target surface, usually in an annular region adjacent to the surface of the target. By so increasing the number and density of electrons trapped in the region nearer the surface of the target, the likelihood that the number of collisions of the electrons with atoms of the inert gas in the space near the target are increased to produce an increase in the number of ions in that region, and thus increasing the useful production of ions. Such increased number of ions are therefore available to be attracted to the target material cathode, resulting in a higher rate of ion bombardment of the target, which upon impact with the target, yields an increased emission rate of sputtering material and thus a more rapid erosion of the target surface. The result is a substantially higher sputtering rate for the same sputtering material than without the aid of a magnetic field.
However, even with magnetic sputtering, the erosion pattern of planar cathode targets results in both low utilization of the target material and uneven target utilization. Thus, although magnetically enhanced sputtering provides improvement in rates of sputtering, the planar target typically erodes through at some point before more than a substantial majority of the total target has sputtered away.
Erosion occurs in a relatively narrow ring shaped region, or a trough, or racetrack shaped region, corresponding to the shape of the closed loop magnetic field. If expensive target materials or targets, either because of their rarity, purity, or difficulty of fabrication, are used, a cost problem arises from the low utilization of the target. Erosion prematurely renders the further use of the target uneconomical, or results in unacceptable non-uniform emission rates and substrate deposition rates. Undesirable erosion patterns on target surfaces can alter the target surface geometry resulting in a departure from the initial emission pattern of the target material which in turn results in undesirable changes in the deposition distribution on the substrate. Efforts to overcome low target utilization involve increasing the area which is significantly eroded before any point erodes all the way through the target.
When other factors which influence the use and selection of target materials are considered and which indirectly influence the flow through or down time of the sputtering process, serious compounded cost problems arise. Since conventional planar magnetron sputtering targets have a target utilization which is approximately 10 to 30%, and because target materials can be very expensive either because of the value of the material or because of the required purity, it is often required to maintain an inventory of targets as well as establish procedures to recover the target materials from scrap targets.
In order to compensate for non-uniform erosion and to increase target utilization, some devices intentionally cause non-uniform emission rates from target surfaces. Reduction in target loss has also been accomplished by moving the target relative to the magnetic field pattern or vice versa by either mechanical or electromechanical methods or techniques. Movement of the magnetic field pattern includes mechanically moving magnetic elements or electrically moving the sputtering plasma by changing electrical fields.
The permissible thickness of cathode targets is limited for a given magnet strength and size for conventional cathode sputtering apparatus because the magnets are placed behind the target and target support whereas a magnetic field of predetermined strength is required in front of the target. Associated with the permissible thickness limitation is a limitation on the amount of sputtering target material and the concomitant constraint in sputtering time before the target must be replaced. Production systems have been used in which substrates are transported on a continuously moving conveyor system underneath and transverse to the magnetron cathode assembly. It is therefore desirable to obtain as high a continuous sputtering time from a target as is possible without having to limit the rate of deposition. When the target material has to be replaced or repositioned within an evacuated deposition chamber, re-evacuation, decontamination, and re-introduction of inert gas is required after each shutdown. Although magnets of increased field strength can be used to apparently overcome the thickness limitation, the problem of low target material percentage utilization still exists.
Some sputtering devices utilize multiple targets with multiple electro-mechanical components, circuitry, and controls which result in generally greater costs, increased manufacturing and maintenance problems, and greater complexity in their operation.
Yet other attempts to further improve the sputtering process include magnetic field shaping structures as well as improved mechanisms to raise and lower the target with respect to the substrate and adjust the angular position of the target with respect to the substrate being coated.
Although these devices and approaches increase the utilization percentage of the target, the increase in cost of the cathode upon which the target is mounted and the increased complexity result in decreased equipment reliability. Such decreased reliability compromises the purpose of the improved magnetically enhanced planar apparatus which can result in a decrease rather than an increase in overall utility.
Further efforts to increase target utilization have led to the development of rotatable magnetron sputtering apparatus. Typically, such devices include a target cathode tube which is rotated around a fixed magnetic array. The tube is mounted in a horizontal position for rotation about its longitudinal axis. The cathode tube has a preformed tubular layer of target material applied to its outer surface and the magnetic means is located within the tube. The cathode tube can be rotated so that each portion of the preformed target affixed to the cathode tube can be rotated into position relative to the substrate for sputtering. Individual preformed target strips, with either the same or different coating materials to be sputtered, can be arranged in a spaced relation and secured to the cathode tube. To provide the requisite heat removal and cooling of the cathode assembly of magnetically enhanced apparatus, a cooling system must also be provided. The deposition rate and permissible target material thickness of low melting point target materials are frequently limited by the rate at which heat can be transferred from the target surface.
Another problem associated with sputtering apparatus and with rotating magnetrons is that the preformed target material has to be affixed to or mounted on the cathode of the rotating cathode or a support affixed to the rotating cathode. Typical means of attaching the target to the cathode or support include bonding, mechanical attachment such as clamping or screwing, and simple mounting by friction or close fit. The attachment must generally be physically strong, make good thermal and electrical contact, and must not contain areas of entrained foreign materials which can contaminate the deposition layer under the evacuated conditions within the chamber. If the target is soldered to a backing plate, it is sometimes very difficult to separate one from the other in order that the unused target material and backing plate can be re-used.
Existing techniques for sputtering liquid materials can be applied for upward deposition on downward-facing substrates utilizing standard planar sputtering cathodes as disclosed in U.S. Pat. No. 3,799,862, issued Mar. 26, 1974 (Krutenat), herein incorporated by reference, and in corresponding German Patent No. 2254434. The technique does not allow downward sputtering onto an upward-facing substrate, as is the preferred orientation for large-area coating systems.